determine convulsive earthquake shocks. Without, however, entering into the difficult question of the causes of the movements, we may inquire into their effects in so far as these register their passing in the annals of geological history.

Their awful suddenness and devastation have invested earthquakes with a high importance in the popular estimate of the forces by which the surface of the globe is modified. Yet if we judge of them by their permanent effects, we must give them a comparatively subordinate place among these forces. After some of the most destructive earthquakes recorded in human history, hardly any trace of the calamity is to be seen, save in shattered and prostrate houses. But when these buildings have been repaired or rebuilt, no one visiting the ground might be able to detect any trace of the earthquake that shattered or overthrew them.

Yet severe earthquakes do not pass without their self-written chronicle which, though often evanescent on the face of nature, is at the time conspicuous enough. Landslips are caused, large masses of earth and blocks of rock being shaken down from higher to lower levels; the ground is rent, and the fissures are sometimes subsequently widened and deepened by rain and runnels into ravines. But more important are the marked changes of level that occasionally accompany earthquake-shocks. In some cases, the ground is raised for several feet, so that along maritime tracts there is a gain of land from the sea; in others, the ground sinks, and the sea flows in upon the land. Yet it is evident that unless these changes are actually witnessed as the accompaniments of the earthquakes, they may take place without retaining any evidence that they were produced by such a cause. The convulsion of an earthquake, notwithstanding the havoc it may bring to the human population of a country, does not always record itself in distinctive and enduring characters in geological history. Some of its most noticeable effects also are not due directly to its own action, but to the operations of the waters of the land and of the sea which, when disturbed by the shock, not infrequently acquire increased vigour in their own peculiar forms of activity. The great waves set in motion by an earthquake roll over the low lands bordering the sea, and may cause vastly more destruction than is done by the mere shock of the earthquake itself.

UPHEAVAL AND SUBSIDENCE.

It is perhaps not so much by earthquakes, as by quiet, hardly perceptible movements, that the relative positions of sea and land are undergoing change at the present time. In some parts of the world the land is gradually rising, in others it is slowly sinking. Proofs of elevation are supplied by lines of barnacles or rockboring shells, now standing above the reach of the highest tides; by caves that have obviously been scooped out by the sea, but now stand at a higher level than the waves can reach; and by deposits of sand, gravel, and shells which were evidently accumulated on a beach, but which now rise above the level where similar materials are now being accumulated (Raised Beaches). Evidences of subsidence are furnished by traces of old landsurfaces- -trees with roots in situ, and beds of peat, lying below the limits of the tides (Submerged Forests). But it must be more difficult to prove subsidence than elevation, for as the land sinks, its surface is carried below the waves, which soon efface the evidence of terrestrial characters.

The time within which man has been observing and recording the changes of the earth's surface forms but an insignificant fraction of the ages through which geological history has been in progress. We cannot suppose that during this brief period he has had experience of every kind of geological process by which the outlines of land and sea are modified. There may be great terrestrial revolutions which happen so rarely that none has occurred since man began to take note of such things. Among these revolutions, of which he has had as yet no experience, the most gigantic is the formation of a mountain-chain. That the various mountain-chains of the globe are of very different ages, and that some of the most gigantic of them are, compared with others, of recent date, are facts in the history of the globe which will be more fully referred to in later pages; but so far as human history or tradition goes, man has never witnessed the uprise of a range of mountains. The crust of the earth has been folded and crumpled on the most colossal scale, some parts having been pushed for miles away from their original position; it has been rent by profound fissures, on each side of which the rocks have been displaced for many thousand feet; and it has been so broken, crushed, and sheared, that its component rocks have in some places assumed a structure entirely different from what they

originally possessed. But of all these colossal mutations there is no human experience. We are driven to reason regarding them from the record of them preserved among the rocks, and from the analogies that can be suggested by experiments devised to imitate as far as possible the processes of nature. To this subject we shall return in Chapter XIII.

Summary.—The enduring records left by volcanoes, whence their former existence in almost all regions of the world may be demonstrated, are to be sought partly in the materials which they have brought up to the surface, and partly in the vents and fissures by which they have discharged these materials. Of the former kind of evidence lava furnishes a conspicuous example; its internal crystalline or glassy structure, its steam-cavities, and the cellular slaggy upper and under parts of the sheets in which it lies, are all proofs of its former molten condition. A succession of lava-beds, piled one above another, marks a series of volcanic eruptions, and the nature of the layers of non-volcanic material intercalated between them may indicate the conditions under which the eruptions took place, whether on land, in lakes, or in the sea. The fragmentary products consolidated into beds of tuff are likewise characteristic of volcanoes; they consist mainly of lava-dust with cindery scoriæ, slags, and blocks; they accumulate most deeply and in coarsest material at and immediately around the volcanic vents, but their finer particles may be carried to enormous distances; they are especially liable to be intercalated with contemporaneous sedimentary deposits in lakes and on the sea-floor.

The vents through which lava and ashes are ejected to the surface form the most permanent record of volcanoes, for, being filled up with volcanic rocks to unknown depths, they cannot be destroyed by the mere denudation of the surface, and can only disappear by being buried under later accumulations. Such "necks" consist sometimes of lava, sometimes of consolidated volcanic debris, or of both kinds of material together, and remain as the stumps of volcanoes, where every other trace of volcanic action may have passed away. Not less enduring are the dykes or wall-like masses of lava which have risen and solidified in open fissures. Enormous sheets of lava appear to have flowed out from such fissures in regions where the volcanic energy never produced any great central cone.

Earthquakes do not impress their mark upon geological history so indelibly as might be supposed. In spite of the destruction which they cause to human life and property, it is by such direct

changes as landslips, rents of the ground, and the upheaval or depression of land, and by such indirect changes as may be produced by derangements of rivers, lakes, and the sea, that earthquakes leave their chief record behind them.

Some of the most important changes of level now going on are effected quietly and almost imperceptibly, some regions being slowly elevated, and others gradually depressed. But the time within which man has been an observer and recorder of nature is too brief to have supplied him with experience of all the ways in which the internal energy of the globe affects its surface. In particular, he has never witnessed the production of a mountainchain, nor any of the plications, fractures, and displacements which the crust of the earth has undergone. Regarding these revolutions we can only reason from the records of them in the rocks, and from such laboratory experiments as may seem most closely to imitate the processes of nature that were concerned in their production.

PART II

ROCKS, AND HOW THEY TELL THE

HISTORY OF THE EARTH

CHAPTER X

THE MORE IMPORTANT ELEMENTS AND MINERALS OF THE EARTH'S CRUST

IN the foregoing Part of this volume we have been engaged in considering the working of various processes by which the surface of the earth is modified at the present time, and some of the more striking ways in which the record of these changes is preserved. We have seen that, on the whole, it is by deposits of some kind, laid down in situations where they can escape destruction, that the story of geological revolution is chronicled. In one place it

is the stalagmite of a cavern, in another the silt of a lake-bottom, in a third the sand and mud of the sea-floor, in a fourth the lava and ashes of a volcano. In these and countless other examples, materials are removed from one place and set down in another, and in their new position, while acquiring novel characters, they retain more or less distinctly the record of their source and of the conditions under which their transference was effected.

In these chapters, reference has intentionally been avoided as far as possible to details that required some knowledge of minerals and rocks, in order that the broad principles of geology, for which such knowledge is not absolutely essential, might be clearly enforced. It is obvious, however, that as minerals and rocks form the records in which the history of the earth has been preserved, this history cannot be followed into detail until some acquaintance with these materials has been made. What now lies before the